There is known a high-frequency differential amplifier circuit for amplifying high-frequency differential signals by using a pair (differential pair) of amplifier transistors. Each amplifier transistor has a gate-drain capacitance (Cgd) between its gate and drain, and there is a problem that high frequency gain is reduced by a negative feedback loop through this capacitance Cgd.
In order to prevent the reduction in gain, it is known to arrange capacitors (cross-coupled capacitors) between the gate of one differential transistor and the drain of the other differential transistor, and between the gate of the other differential transistor and the drain of one differential transistor. These capacitors are arranged to cancel the reduction in high frequency gain due to the negative feedback loop. The capacitor is made of a metal material, for example. The route from the drain of the other or one differential transistor to the gate of one or the other differential transistor through the capacitor is called a positive feedback loop or a neutralization loop.
Now the impedance of the negative feedback loop and that of neutralization loop will be considered. The impedance of the negative feedback loop has a real component supplied by a gate resistance Rg, while the impedance of the neutralization loop has only an imaginary component supplied by the capacitor. Accordingly, it is impossible to completely neutralize the negative feedback of the amplifier transistors only with the cross-coupled capacitors.
The capacitances of the cross-coupled capacitors are both represented as Cf. When the capacitance Cf is smaller than the capacitance value Cgd of the amplifier transistor, a stability coefficient K of the amplifier circuit becomes larger as the capacitance Cf is made larger (that is, the stability of the amplifier circuit is increased). This is because total signal feedback amount (total of the feedback amount of the neutralization loop and the feedback amount of the negative feedback loop) becomes smaller as the capacitance Cf becomes larger.
When the value of the capacitance Cf and that of the capacitance Cgd are nearly the same, the total signal feedback amount becomes nearly zero, and the stability coefficient K has the maximum value. Note that K cannot be infinite since it is impossible in the above structure to completely eliminate the total signal feedback amount as stated above. Further, when the capacitance Cf is made larger, total positive feedback amount is increased and the stability coefficient K becomes smaller (that is, the stability of the amplifier circuit is reduced).
Note that the circuit is stable when the value of the stability coefficient K is equal to or greater than 1, and the stability is increased as the value of K becomes larger. The circuit is not stable when the value of K is less than 1, and the stability is reduced as the value becomes smaller. When K is 1, the maximum gain can be obtained without causing oscillation.
At the design stage, a great gain design value can be obtained by setting the design value of the capacitance Cf so that K is around 1 as nearly as possible, which leads to a great possibility that the amplifier oscillates when there is a gap between the actual capacitance Cf and the design value due to manufacturing variation (the stability coefficient K becomes smaller than 1). Thus, at the design stage, the capacitance Cf must be determined considering the scope of the stability, and it is difficult to realize a design for obtaining the maximum gain.